OPTICAL INSTRUMENTS; 



17 



Fig. 23. 



should be formed by the revolution of 

 the hyperbolic curve : this speculum has 

 a small hole in its centre. E is another 

 concave elliptical small metal placed in 

 the axis of the larger one, at a distance 

 from it a little more than the sum of their 

 focal distances. H are the eye-lenses 

 sliding in a tube fixed behind the large 

 speculum : the adjustment is made by 

 the screw s s, which moves the small 

 metal to or from the great speculum. 

 Let r B and r D be two parallel rays 

 from a distant object, these will be re- 

 flected to the focus F of the large metal, 

 where an image will be formed, and the 

 rays, crossing each other, fall upon the 

 small speculum E ; and if the focus of 

 this metal had coincided with the focus F, 

 the rays would have been reflected pa- 

 rallel, but now they form a direct image 

 at I, and this image is viewed by the 

 eye-piece or a single lens at H. The 

 magnifying power of this instrument 

 may be computed thus : suppose the 

 focus of the large speculum B F is 9 

 inches, and the focus of the small metal 

 1 4 inch : then will the angle be in- 

 creased six times ; but this must be 

 multiplied by the ratio of the distances 

 I H, the focus of the lens, and the dis- 

 tance I F ; and if these are as 1 to 8, 

 the amplification of the object will be 

 6 x 8 = 48 times. 



(29.) The Cassegrainian reflector is 

 constructed in the same way as the 

 Gregorian, with the exception of a small 

 convex spherical speculum, instead of 

 one a little concave ; and as the focus of 

 this metal is negative, it is placed at a 

 distance from the larger metal, equal to 

 the difference of their foci, and only one 

 image is formed, viz. that in the focus 

 of the eye-glass ; on this account, 

 the distinctness is considerably greater 

 than in the Gregorian. Mr. Ramsden, 

 in the G9th volume of the Philosophical 

 Transactions, states, that this construc- 

 tion is preferable to either of the former 

 reflectors, because the aberrations of the 

 two metals have a tendency to correct 

 each other ; whereas in the Gregorian, 

 both the metals being concave, any 

 error in the specula will be doubled. 



By assuming such proportions of the 

 foci of the specula, as are generally 

 employed in these instruments,which are 

 about as 1 to 4, he asserts that the 

 aberration or indistinctness occasioned 

 by the figures of the reflectors (sup- 

 posing each worked equally true) in the 

 Cassegrainian construction, is to that in 

 the Gregorian as 3 to 5. 



(30.) In sidereal observations of ne- 

 bulae and small stars, abundance of 

 light is necessarily required, and by 

 whatever means a loss of light by re- 

 flection or refraction can be prevented, 

 the adoption of such a construction 

 would be advisable. Sir W. Herschel, 

 from an investigation of the loss of light 

 occasioned by the small speculum in 

 reflectors, constructed an instrument 

 which entirely obviated the use of the 

 second metal, by what he called the 

 front view telescope. The diameter of 

 the polished surface of the speculum of 

 his large instrument was 4 8 inches, and 

 its focal distance 40 feet. This metal, 

 which weighed when taken from the 

 casting, 2118 lb., was placed at the end 

 of an iron tube 4 feet 1 inches in dia- 

 meter ; the other end is elevated towards 

 the object, and has attached to it a single 

 eye-lens in the focus of the metal ; the. 

 observer is mounted in a gallery move- 

 able with the instrument, having his 

 back to the object. The light obtained 

 from so large a surface by this instru- 

 ment was truly surprising, and enabled 

 objects otherwise invisible to become 

 extremely interesting. This telescope, 

 erected at Slough, near Windsor, was 

 completed on the 28th of August, 

 1789, and on the same day the sixth 

 satellite of Saturn was discovered*. 

 The frame of this instrument having 

 greatly decayed, it has been taken down ; 

 and another, of 20 feet focus and 18 

 inches diameter, erected in its place, 

 by his son Mr. J. Herschell, in 1822. 



* A full description of this instrument will be 

 found in the Transactions of the Royal Society for 

 1793, explained by means of 18 plates and 63 pages 

 of letter-press ; and an ample detail is given of every 

 circumstance relating to the mechanical construc- 

 tion of this instrument. 



